Distortion compensating circuit having negative gain deviation, power amplifier using the same, and communication device having power amplifier

ABSTRACT

There is used a distortion compensating circuit having a structure in which a circuit including a diode is connected between a signal path and a ground, wherein a voltage applied to the diode is a forward bias voltage, and an amount of change in a ratio between output power output from the signal path and input power input to the signal path with respect to the input power is zero or negative.

This nonprovisional application is based on Japanese Patent ApplicationNo. 2004-344591 filed with the Japan Patent Office on Nov. 29, 2004, theentire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a distortion compensating circuit, apower amplifier using the same, and a communication device having thepower amplifier. In particular, the present invention relates to adistortion compensating circuit used in a wireless communication deviceor the like requiring low-distortion amplification, a power amplifierusing the same, and the communication device.

2. Description of the Background Art

In current wireless communication systems such as a wireless LAN (LocalArea Network) system and a mobile phone, a dominant technology isdigital modulation and demodulation such as OFDM (Orthogonal FrequencyDivision Multiplex) and QPSK (Quadrature Phase Shift Keying). In thesedigital modulation and demodulation schemes, information is carried byusing both a phase and an amplitude of a signal, and thus a poweramplifier is required to linearly amplify an input signal. Further, in apower amplifier for transmission consuming much of power consumption ofthe system during transmission, the power amplifier is also required tooperate with high efficiency in order to achieve system miniaturizationand low power consumption.

As a technique for enhancing linearity of a power amplifier, there hasbeen proposed a method of canceling distortion by providing, at apreceding stage or a subsequent stage of a power amplifying element usedin a power amplifier, a distortion compensating circuit havingnonlinearity contrary to the power amplifying element. As an example ofa conventional distortion compensating circuit, a technique forconnecting a circuit including a diode between a signal path and aground has been disclosed (see for example Japanese Patent Laying-OpenNo. 2001-144550).

FIG. 9 is a circuit block diagram of a conventional distortioncompensating circuit 100.

Referring to FIG. 9, conventional distortion compensating circuit 100has an input terminal 101, capacitors 102, 103 and 110, an outputterminal 104, a diode 105, resistors 106 and 108, and a direct-currentpower source 109.

Diode 105 has an anode terminal electrically coupled to a connectingnode between capacitor 102 and capacitor 103, and a cathode terminalgrounded via resistor 106. Direct-current power source 109 is connectedto the anode. terminal of diode 105 via resistor 108, and a forward biasvoltage is applied to diode 105. Capacitor 110 is connected to aconnecting node between resistor 108 and direct-current power source109, and the connecting node is grounded via capacitor 110.

Hereinafter, an operation of conventional distortion compensatingcircuit 100 will be described.

A signal input from input terminal 101 is output to output terminal 104via capacitors 102 and 103. However, a portion of the signal leaks toground via diode 105 and resistor 106 connected between capacitor 102and capacitor 103, and via resistor 108 and capacitor 110. Since anincrease in the input signal leads to increased power of the signalinput to diode 105, a flow of direct current is produced by therectification function of the diode, and a bias point of the diodemoves.

FIG. 10 illustrates the movement of the bias point in conventionaldistortion compensating circuit 100.

Referring to FIG. 10, a curve 111 indicates a current-voltagecharacteristic of a diode when a signal is not input. When an inputsignal is small, the diode operates on a small-signal bias point S′ oncurve 111. On the other hand, when input power is increased, a flow ofdirect current is produced by the rectification function of the diode asdescribed above, and the current-voltage characteristic of the diodechanges from curve 111 to a curve 112.

As a result, the bias point moves from small-signal bias point S′ to alarge-signal bias point L′, along a load line 113 determined byresistors 106 and 108.

A resistance component of a diode with respect to high-frequency poweris represented by an RF (radio frequency) resistance value, whichcorresponds to the reciprocal of the slope of a tangent to the curveindicating a current-voltage characteristic at a bias point. When powerof a signal input to the diode is increased, the bias point moves frompoint S′ to point L′, and the tangent has a gentler slope, as shown inFIG. 10. That is, the RF resistance value of the diode increases.Accordingly, the amount of signal power leak to diode 105 is reducedwith the increase in the input power, and as a result, output power fromoutput terminal 104 increases with the increase in the input power. FIG.11 shows this result.

FIG. 11 illustrates relationship between input power and gain, that is,power ratio between an output signal and an input signal, of aconventional distortion compensating circuit.

As shown in FIG. 11, the amount of gain reduction decreases with anincrease in the input power. This indicates that the distortioncompensating circuit has positive gain deviation, that is, it has afunction of suppressing negative gain deviation. The positive gaindeviation in the distortion compensating circuit can be adjusted by apower supply voltage of direct-current power source 109, which has beendisclosed for example in Japanese Patent No. 3335907. The negative gaindeviation can be suppressed by reducing the power supply voltage asshown in FIG. 11.

It is generally know that, to operate a power amplifier with highefficiency, a direct bias current in a power amplifying element used inthe power amplifier should be reduced.

If the direct bias current is reduced, however, gain of the poweramplifying element tends to increase with an increase in output powerbefore the output power is saturated.

FIG. 12 illustrates relationship between output power and both gain andpower efficiency of a power amplifying element.

As shown in FIG. 12, gain variation associated with an increase in theoutput power means that the linearity of a power amplifier isdeteriorated. FIG. 12 indicates that improving the power efficiency andimproving the linearity are in trade-off relationship.

Therefore, in order to operate a power amplifier with high linearity andwith high efficiency, a distortion compensating circuit is requiredwhich can suppress gain increase caused when a direct bias current in apower amplifying element is reduced, that is, positive gain deviation.

However, since a distortion compensating circuit according to the aboveconventional technique has positive gain deviation, there has been aproblem that it cannot be applied as a distortion compensating circuitfor a power amplifying element having positive gain deviation.

SUMMARY OF THE INVENTION

The present invention has been made to solve the problem in aconventional distortion compensating circuit as described above. Oneobject of the present invention is to provide a distortion compensatingcircuit capable. of suppressing positive gain deviation in an poweramplifying element caused by reducing a direct bias current for highlyefficient operation, that is, a distortion compensating circuit havingnegative gain deviation, a power amplifier capable of achievingoperation with high efficiency and with high linearity simultaneously byusing the same, and a communication device having the power amplifier.

A distortion compensating circuit in accordance with the presentinvention has a structure in which a circuit including a diode isconnected between a signal path and a ground. A voltage applied to thediode is a forward bias voltage, and an amount of change in a ratiobetween output power output from the signal path and input power inputto the signal path with respect to the input power is zero or negative.

Preferably, the voltage applied to the diode is a forward bias voltagehaving a current density of a direct bias current flowing to the diodeof not more than 10² A/cm².

According to the distortion compensating circuit in accordance with thepresent invention, the voltage applied to the diode is a forward biasvoltage, and the amount of change in the ratio between output poweroutput from the signal path and input power input to the signal pathwith respect to the input power is zero or negative. With thisarrangement, a bias point of the diode moves with an increase in theinput power to reduce an RF resistance value of the diode, and as aresult, a distortion compensating circuit in which output powerdecreases with an increase in input power, that is, a distortioncompensating circuit having negative gain deviation, can be provided.Therefore, since an operation with high efficiency and high linearity isperformed with a simple structure, miniaturization, reduced powerconsumption, and even cost reduction can be achieved.

A power amplifier in accordance with the present invention includes apower amplifying element having positive gain deviation with an increasein output power, and the distortion compensating circuit describedabove.

Preferably, the distortion compensating circuit is provided in apreceding stage of the power amplifying element.

Preferably, the power amplifying element is formed of a bipolartransistor.

In particular, the diode is made of at least one of an emitter-basejunction and a base-collector junction of the bipolar transistor.

Preferably, the distortion compensating circuit is provided at least ina preceding stage of the power amplifying element in a final outputstage of a multi-stage power amplifier including a plurality of thepower amplifying elements.

Since the power amplifier in accordance with the present inventionincludes the distortion compensating circuit described above,miniaturization, reduced power consumption, and even cost reduction ofthe power amplifier can be achieved.

A communication device in accordance with the present invention includesthe power amplifier described above.

Preferably, the power amplifier is a power amplifier for transmission.

Since the communication device in accordance with the present inventionincludes the power amplifier described above, miniaturization, reducedpower consumption, and even cost reduction of the communication devicecan be achieved, as well as extended communication duration and batteryminiaturization.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit block diagram of a distortion compensating circuitin accordance with a first embodiment of the present invention.

FIG. 2 illustrates movement of a bias point in the distortioncompensating circuit in accordance with the first embodiment of thepresent invention.

FIG. 3 illustrates relationship between input power and gain of thedistortion compensating circuit in accordance with the first embodimentof the present invention, as well as relationship between input powerand gain in a first comparative example.

FIG. 4 is a circuit block diagram of a power amplifier in accordancewith a second embodiment of the present invention.

FIG. 5 illustrates relationship between output power and gain of thepower amplifier in accordance with the second embodiment of the presentinvention, gain of a power amplifier in a second comparative example notusing a distortion compensating circuit, and gain of a conventionalpower amplifier.

FIG. 6 is a circuit block diagram of a power amplifier in accordancewith a third embodiment of the present invention.

FIG. 7 illustrates relationship between current consumption and outputpower in the power amplifier in accordance with the third embodiment andin a power amplifier in a third comparative example.

FIG. 8 is a schematic block diagram showing a structure of main units ofa communication device in accordance with a fourth embodiment of thepresent invention.

FIG. 9 is a circuit block diagram of a conventional distortioncompensating circuit.

FIG. 10 illustrates movement of a bias point in the conventionaldistortion compensating circuit.

FIG. 11 illustrates relationship between input power and gain, that is,power ratio between an output signal and an input signal, of aconventional distortion compensating circuit.

FIG. 12 illustrates relationship between output power and both gain andpower efficiency of a power amplifying element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings. It is to be noted that identicalor corresponding parts in the drawings will be designated by the samereference numerals, and the description thereof will not be repeated.

First Embodiment

Referring to FIG. 1, a distortion compensating circuit 20 in accordancewith a first embodiment of the present invention has an input terminal1, capacitors 2, 3 and 10, an output terminal 4, a diode 5, resistors 6and 8, an inductor 7, and a direct-current power source 9.

Diode 5 has an anode terminal electrically coupled to a connecting nodebetween capacitor 2 and capacitor 3, and a cathode terminal grounded viaresistor 6. Direct-current power source 9 is connected to the anodeterminal of diode 5 via inductor 7 and resistor 8, and a forward biasvoltage of not more than a turn-on voltage V_(on) is applied to diode 5.

A connecting node between resistor 8 and direct-current power source 9is grounded via capacitor 10. It is to be noted that diode 5 isconnected so that the forward bias voltage is applied thereto. Forexample, diode 5 may also be structured such that the cathode terminalis connected to the connecting node between capacitor 2 and capacitor 3and the direct-current power source is connected to the anode terminalside. Further, a resistor or an inductor may be added or removed asappropriate, depending on design specification of the circuit.

Hereinafter, an operation of distortion compensating circuit 20 inaccordance with the first embodiment of the present invention will bedescribed.

A signal input from input terminal 1 is output to output terminal 4 viacapacitors 2 and 3. However, a portion of the signal leaks to ground viadiode 5 and resistor 6 connected between capacitor 2 and capacitor 3,and via inductor 7, resistor 8 and capacitor 10.

Since an increase in the input signal leads to increased power of thesignal input to diode 5, a flow of direct current is produced by therectification function of the diode. The direct current produced by therectification function, that is, an increased amount of the directcurrent, varies depending on the magnitude of the power of the signalleaking to the diode, that is, high-frequency power, and a bias point ofthe diode, as described above. This results from the fact that the diodehas a nonlinear operational characteristic represented by the followingexpression:Id=Is·{exp(qV _(d) /kT)−1},where Id is a current in the diode, Is is a reverse saturation currentin the diode, q is the amount of elementary charge (≈1.6×10⁻¹⁹C), k isBoltzmann constant (≈1.38×10⁻²³J·K⁻¹), T is the absolute temperature,and V_(d) is a direct bias voltage applied to the diode.

When high-frequency power having a voltage amplitude v_(in) and anangular frequency ω is input to the diode, the current flowing to thediode is represented by the following expression:Id≈Is·exp{q(V _(d) +v _(in) cos(ωt))/kT},where t is time.

If the above expression is used to determine the direct current flowingto the diode when the high-frequency power is input, the followingexpression is obtained:Id _(dc) =Is ·exp(qV _(d) /kT)·I ₀(qv _(in) /kT),where function I₀(x) is the zero-order modified Bessel function of thefirst kind, which is a monotonic increasing function with respect to x.

From the above expression, it can be understood that the direct currentflowing to the diode increases with an increase in voltage V_(d) appliedto the diode, and with an increase in voltage amplitude vin input to thediode.

Referring to FIG. 2, explanation will be given on movement of the biaspoint in distortion compensating circuit 20 in accordance with the firstembodiment of the present invention. The current-voltage characteristicof the diode to which a large signal is input changes from a curve 11 toa curve 12 as shown in FIG. 2, and the bias point of the diode moves.

In FIG. 2, V_(on) is the turn-on voltage. In the structure in accordancewith the first embodiment, the bias voltage for the diode is set at notmore than turn-on voltage V_(on). A point S is a small-signal bias pointin the first embodiment. A point L is a large-signal bias point in thefirst embodiment.

In contrast, a point S′ and a point L′ indicate a small-signal biaspoint and a large-signal bias point, respectively, when the bias voltagefor the diode is set at more than turn-on voltage V_(on), as a firstcomparative example.

The diode in accordance with the first embodiment is for example a GaAsPIN diode having a turn-on voltage of 1.2 V. In the first embodiment,the bias voltage applied to the diode and the current density of thedirect bias current are set at 1.0 V and 1 A/cm², respectively.

In contrast, in the first comparative example, the bias voltage appliedto the diode and the current density of the direct bias current are setat 1.3 V and 5×10² A/cm², respectively.

When an input signal is small, the diode operates on small-signal biaspoints S and S′ on curve 11. When input power is increased, a flow ofrectification current is produced, changing the current-voltagecharacteristic of the diode. The bias points move from small-signal biaspoints S and S′ on curve 11 to large-signal bias points L and L′ oncurve 12, respectively, along load lines 13 and 14 determined byresistors 6 and 8.

In the first embodiment in which the bias voltage for the diode is notmore than turn-on voltage V_(on), the slope of a tangent to the curveindicating the current-voltage characteristic at the bias pointincreases with an increase in input power. Specifically, the RFresistance value of the diode decreases with the increase in the inputpower. Accordingly, the amount of signal power leak to diode 5 increaseswith the increase in the input power, and as a result, output power fromoutput terminal 4 decreases with the increase in the input power.

In contrast, in the first comparative example in which the bias voltagefor the diode is more than turn-on voltage V_(on), the slope of atangent to the curve indicating the current-voltage characteristic atthe bias point decreases with an increase in input power. Specifically,the RF resistance value of the diode increases with the increase in theinput power. Accordingly, the amount of signal power leak to diode 5decreases with the increase in the input power, and as a result, outputpower from output terminal 4 increases with the increase in the inputpower.

Referring to FIG. 3, explanation will be given on relationship betweeninput power and gain of the distortion compensating circuit inaccordance with the first embodiment of the present invention, as wellas relationship between input power and gain in the first comparativeexample.

As shown in FIG. 3, in the first embodiment of the present invention,the amount of gain reduction increases with an increase in input power.This indicates that the distortion compensating circuit in accordancewith the first embodiment has negative gain deviation, that is, it has afunction of suppressing positive gain deviation. It is to be noted that,although the bias voltage applied to the diode and the current densityof the direct bias current are set at 1.0 V and 1 A/cm², respectively,in the present embodiment, a similar effect can be obtained if thevoltage applied to the diode is a forward bias voltage having a currentdensity of the direct bias current of not more than 10² A/cm².Specifically, a distortion compensating circuit having negative gaindeviation with minimum power consumption in a diode can be achieved bysetting a voltage applied to the diode at a forward bias voltage havinga current density of a direct bias current flowing to the diode of notmore than 10² A/cm².

As described above, in distortion compensating circuit 20 in accordancewith the first embodiment of the present invention, the voltage appliedto the diode is set at a forward bias voltage of not more than theturn-on voltage such that the amount of change in the ratio betweenoutput power output from a signal path and input power input to thesignal path with respect to the input power is zero or negative, andthus the bias point of the diode moves with the increase in the inputpower, reducing the RF resistance value of the diode. Accordingly, theamount of signal power leak to the diode can be increased with theincrease in the input power. As a result, a distortion compensatingcircuit in which output power decreases with an increase in input power,that is, a distortion compensating circuit having negative gaindeviation, can be provided.

It is to be noted that, although a GaAs PIN diode is used as the diodein the present embodiment, a PIN diode using another semiconductorelement such as Si or InP can also be used. Further, other than a PINdiode, a pn junction diode or a Schottky junction diode can also beapplied.

Second Embodiment

Referring to FIG. 4, a power amplifier 50 in accordance with a secondembodiment of the present invention includes an input terminal 41, apower amplifying element 42, an output terminal 43, an input matchingcircuit 44, a bias circuit 45 on the input side of power amplifyingelement 42, a power supply circuit 46 on the output side of poweramplifying element 42, an output matching circuit 47, and a distortioncompensating circuit 48.

Distortion compensating circuit 48 is equivalent to distortioncompensating circuit 20 described in FIG. 1.

Power amplifying element 42 is formed of a GaAs HBT (HeterojunctionBipolar Transistor), which is a bipolar transistor, and distortioncompensating circuit 48 is provided in a preceding stage of poweramplifying element 42.

A diode 49 in distortion compensating circuit 48 is formed of a Ti/GaAsSchottky diode having a turn-on voltage of 0.6 V. A bias voltage appliedto the diode and a current density of a direct bias current are set at0.5 V and 1 A/cm², respectively. Specifically, the bias voltage appliedto the diode is set at not more than the turn-on voltage. The directbias current in power amplifying element 42 is reduced for highlyefficient operation, and power amplifying element 42 operates in a statewhere gain increases with an increase in output power before the outputpower is saturated, that is, in a state where positive gain deviationoccurs.

Referring to FIG. 5, explanation will be given on relationship betweenoutput power and gain of the power amplifier in accordance with thesecond embodiment of the present invention, gain of a power amplifier ina second comparative example not using a distortion compensatingcircuit, and gain of a conventional power amplifier.

As shown in FIG. 5, in the power amplifier in accordance with the secondembodiment of the present invention, gain deviation is suppressed untilhigher output power, when compared to the power amplifier in the secondcomparative example not using a distortion compensating circuit and theconventional power amplifier. As a result, linear output of the poweramplifier is improved, implementing a power amplifier achievingoperation with high linearity and high efficiency.

As described above, according to the power amplifier in accordance withthe second embodiment of the present invention, even when the directbias current in the power amplifying element is reduced to a level inwhich positive gain deviation occurs for highly efficient operation, thepositive gain deviation can be cancelled by negative gain deviation ofthe distortion compensating circuit. Accordingly, gain deviation of thepower amplifier can be suppressed, improving linear output of the poweramplifier. Thus, a power amplifier achieving operation with highlinearity and high efficiency can be provided.

It is to be noted that, although the power amplifier in accordance withthe second embodiment of the present invention has been described tohave distortion compensating circuit 48 in the preceding stage of poweramplifying element 42, the structure of the power amplifier is notlimited to the one described above, and it may also be formed to havedistortion compensating circuit 48 in a subsequent stage of poweramplifying element 42.

When distortion compensating circuit 48 is provided in the precedingstage of power amplifying element 42, signal power level in distortioncompensating circuit 48 is lower than that in the structure havingdistortion compensating circuit 48 in the subsequent stage of the poweramplifying element. Accordingly, efficiency reduction due to power lossby distortion compensating circuit 48 can be minimized.

Further, although distortion compensating circuit 48 is provided to bedirectly connected to the preceding stage of power amplifying element 42in the present embodiment, the structure of the power amplifier is notlimited to the one described above, and it may also be formed such thatfor example a portion of input matching circuit 44 is inserted betweendistortion compensating circuit 48 and the preceding stage of poweramplifying element 42.

Furthermore, although a GaAs HBT is used as the power amplifying elementin the present embodiment, another bipolar transistor such as a Sibipolar transistor, a SiGe HBT, or a InP HBT may also be used forimplementation.

Further, although a bipolar transistor is used as the power amplifyingelement in the present embodiment, an FET (Field-Effect Transistor) maybe applied as the power amplifying element.

It is to be noted that a power amplifier with linearity and efficiencyimproved more effectively can be designed by using a bipolar transistoras a power amplifying element. This is because, since positive gaindeviation of the power amplifying element which occurs when the directbias current is reduced occurs associated with the movement of the biaspoint of the power amplifying element caused by an increase in inputpower, a bipolar transistor in which the direct bias current changesexponentially with respect to a base bias voltage has greater positivegain deviation than that of a field effect transistor in which thedirect bias current changes in proportion to the square root of a gatebias voltage.

Further, although a Ti/GaAs Schottky diode is used as the diode in thepresent embodiment, a Schottky diode using another metal/semiconductorjunction such as Al/Si can also be applied. Furthermore, other than aSchottky diode, a pn junction diode or a PIN diode can also be used forimplementation.

Third Embodiment

Referring to FIG. 6, a power amplifier 60 in accordance with a thirdembodiment of the present invention has a multi-stage structure usingtwo stages of power amplifying elements. Power amplifier 60 has an inputterminal 61, power amplifying elements 62 and 72, an input matchingcircuit 64, bias circuits 65 and 75 on the input side of poweramplifying elements 62 and 72, respectively, power supply circuits 66and 76 on the output side of power amplifying elements 62 and 72,respectively, a distortion compensating circuit 68, interstage matchingcircuits 70 and 71, an output terminal 73, and an output matchingcircuit 77.

Power amplifying elements 62 and 72 are formed of GaAs HBTs, which arebipolar transistors. A diode 69 in distortion compensating circuit 68 isformed of a pn junction diode made of a base-collector junction of aGaAs HBT. By forming diode 69 on the same semiconductor chip as thepower amplifying elements, miniaturization and cost reduction of a poweramplifier can be achieved. It is to be noted that distortioncompensating circuit 68 is equivalent to distortion compensating circuit20 described in FIG. 1 except that a resistor 79 is additionallyprovided.

Distortion compensating circuit 68 is provided in a preceding stage ofpower amplifying element 72 in the final stage of the multi-stage poweramplifier, via interstage matching circuit 71. Resistor 79 as a biascircuit for setting the voltage applied to the diode at a forward biasvoltage of not more than a turn-on voltage is connected in parallel withthe diode.

The direct bias current in power amplifying element 72 in the finalstage is reduced for highly efficient operation, and power amplifyingelement 72 operates in a state where gain increases with an increase inoutput power before the output power is saturated, that is, in a statewhere positive gain deviation occurs.

Referring to FIG. 7, explanation will be given on relationship betweencurrent consumption and output power in the power amplifier inaccordance with the third embodiment and in a power amplifier in a thirdcomparative example. It is to be noted that a power amplifier not usingdistortion compensating circuit 68 in the structure in accordance withthe third embodiment will be described as the power amplifier in thethird comparative example. In this case, the power amplifier in thethird comparative example should have a greater direct bias current thanthat in the power amplifier in accordance with the third embodiment ofthe present invention to obtain the same linearity as that in the thirdembodiment. With this arrangement, the power amplifier in the thirdcomparative example operates to obtain the same linearity.

Here, the direct bias current in the power amplifying element in thefinal stage in the third comparative example is about three times thatin the third embodiment. As can be seen from FIG. 7, in comparison tothe third comparative example in which high linearity is obtained byincreasing the direct bias current without using a distortioncompensating circuit, the power amplifier in accordance with the thirdembodiment of the present invention consumes less current throughout therange of the output power, implementing a power amplifier achievingoperation with high linearity and high efficiency.

As described above, according to the power amplifier in accordance withthe third embodiment of the present invention, even when the direct biascurrent in the power amplifying element is reduced to a level in whichpositive gain deviation occurs for highly efficient operation, thepositive gain deviation can be cancelled by negative gain deviation ofthe distortion compensating circuit, and gain deviation of the poweramplifier can be suppressed. Consequently, a power amplifier capable ofreducing power consumption while maintaining linear output of the poweramplifier can be provided.

Further, according to the power amplifier in accordance with the thirdembodiment of the present invention, since the diode in the distortioncompensating circuit is formed of a pn junction diode made of abase-collector junction (or an emitter-base junction) of a GaAs HBT, thediode can be formed on the same semiconductor chip as the poweramplifying elements, and thus miniaturization and cost reduction of thepower amplifier can be achieved.

It is to be noted that, although the distortion compensating circuit isprovided only in the preceding stage of the power amplifying element inthe final stage in the power amplifier in accordance with the presentembodiment, it is of course possible that the distortion compensatingcircuit may be provided in a preceding stage or a subsequent stage ofeither power amplifying element.

Further, the distortion compensating circuit can be provided in apreceding stage of the power amplifying element in the final outputstage of a multi-stage power amplifier including a plurality of thepower amplifying elements. Since the multi-stage power amplifier has thegreatest output at the power amplifying element in the final outputstage, which exerts the greatest effect on the efficiency of the poweramplifier, linearity and efficiency can be improved more effectivelywhen the distortion compensating circuit is provided in the precedingstage of the power amplifying element in the final output stage.

Furthermore, although the distortion compensating circuit is provided inthe preceding stage of the power amplifying element in the final stagevia the interstage matching circuit, the distortion compensating circuitcan also be provided to be directly connected with the preceding stageof the power amplifying element.

Further, although explanation has been given on the diode in the presentembodiment using a pn junction diode made of a base-collector junctionof a bipolar transistor, another diode such as a pn junction diode madeof an emitter-base junction may also be used for implementation.

Furthermore, although a GaAs HBT is used as the power amplifying elementin the present embodiment, another bipolar transistor such as a Sibipolar transistor, a SiGe HBT, or a InP HBT may also be used.

Further, although the power amplifier in the present embodiment has beendescribed as a multi-stage structure using two stages of the poweramplifying elements, the present embodiment may also be applied to amulti-stage structure using three or more stages of the power amplifyingelements.

Fourth Embodiment

Referring to FIG. 8, a communication device 80 in accordance with afourth embodiment of the present invention has, as main units, an RFunit for transmission 81 including a power amplifier 82 with themulti-stage structure described in the third embodiment of the presentinvention, a filter 83, a driver amplifier 84, and the like; an RF unitfor reception 85 including a low-noise amplifier 86, a filter 87, andthe like; a frequency converting unit 88 converting between an RF signaland an IF (intermediate frequency) signal; an IF/baseband unit 89; aswitch 90; and an antenna 91. Power amplifier 82 with the multi-stagestructure has a distortion compensating circuit 93 provided in apreceding stage of a power amplifying element 92 in the final stage.

Since RF unit for transmission 81 including driver amplifier 84, poweramplifier 82, and the like handles the greatest signal power withincommunication device 80, the amplifier consumes large power anddistortion is likely to occur during amplification. In the fourthembodiment, however, by using the power amplifier described above withinRF unit for transmission 81, a transmission signal can be amplified toreach a predetermined antenna output with high linearity and low powerconsumption, achieving reduced power consumption of communication device80.

Further, since an amplifier stage closer to antenna 91 amplifies greatersignal power, it is more effective to use the power amplifier of thepresent invention as an amplifier closer to the antenna in order toachieve lower power consumption of communication device 80. Furthermore,a duration for communication until a battery is exhausted can beextended, which is important for a battery-driven mobile terminal usinga lithium-ion battery, a nickel-hydride battery, or the like representedby a mobile phone as communication device 80. Further, if the durationfor communication is the same as a conventional duration, asmaller-sized battery can be used, implementing a smaller and lightercommunication device.

Furthermore, when the communication device in accordance with the fourthembodiment of the present invention is used in a communication systemrequiring adjacent channel leakage power and stringent low-noisecharacteristic represented by the EVM (Error Vector Magnitude) standardfor a power amplifier for transmission, such as a wireless LAN compliantwith W-CDMA (Wideband Code Division Multiple Access), IEEE802.11a, orthe like, reduction in distortion and increase in efficiency in thepower amplifier for transmission can be achieved simultaneously, andthus a communication device having a sufficient effect, that is, acommunication device with smaller size and reduced power consumption,can be obtained.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1. A distortion compensating circuit having a structure in which acircuit including a diode is connected between a signal path and aground, wherein a voltage applied to said diode is a forward biasvoltage, and an amount of change in a ratio between output power outputfrom the signal path and input power input to the signal path withrespect to the input power is zero or negative.
 2. The distortioncompensating circuit according to claim 1, wherein the voltage appliedto said diode is a forward bias voltage having a current density of adirect bias current flowing to said diode of not more than 10²A/cm². 3.A power amplifier, comprising: a power amplifying element havingpositive gain deviation with an increase in output power, and adistortion compensating circuit having a structure in which a circuitincluding a diode is connected between a signal path and a ground,wherein a voltage applied to said diode is a forward bias voltage, andan amount of change in a ratio between output power output from thesignal path and input power input to the signal path with respect to theinput power is zero or negative.
 4. The power amplifier according toclaim 3, wherein said distortion compensating circuit is provided in apreceding stage of said power amplifying element.
 5. The power amplifieraccording to claim 3, wherein said power amplifying element is formed ofa bipolar transistor.
 6. The power amplifier according to claim 5,wherein said diode is made of at least one of an emitter-base junctionand a base-collector junction of said bipolar transistor.
 7. The poweramplifier according to claim 3, wherein said distortion compensatingcircuit is provided at least in a preceding stage of the poweramplifying element in a final output stage of a multi-stage poweramplifier including a plurality of the power amplifying elements.
 8. Acommunication device, comprising a power amplifier, wherein said poweramplifier includes: a power amplifying element having positive gaindeviation with. an increase in output power, and a distortioncompensating circuit having a structure in which a circuit including adiode is connected between a signal path and a ground, a voltage appliedto said diode is a forward bias voltage, and an amount of change in aratio between output power output from the signal path and input powerinput to the signal path with respect to the input power is zero ornegative.
 9. The communication device according to claim 8, wherein saidpower amplifier is a power amplifier for transmission.